We have obtained a human x mouse hybridoma, C3A32 produced by using human lymphocytes immunized in vitro with tetanus toxoid, then fused with mouse myeloma line. The hybridoma secretes human IgM antitetanus antibody. Cells of clone C3A2 have the following characteristics: (1) they stain with both labeled anti-IgG and anti-IgM. The anti-IgG is blocked by Hu IgG only; the anti-IgM is blocked by IgM only. In ELISA assays most clones of C3A2 produce anti-tetanus antibody with antigenic characteristics of both IgG and IgM. The immunoglobulin produced appears to be a single species of macroglobulin (IgM) with some antigenic characteristics of IgG not founded in human serum IgM.